6) Clipping wing feathers is no different than cutting your hair or toenails, not painful to the birds, but stressful if they aren't used to being handled. Clipping both wings will restrict flight more than just clipping one. Whether just one is sufficient would likely depend the breed, some chickens are far better at flying (and escaping) than others.
8) Rate for POL pullet depends your area, the breed, and if it is breeding/show quality or not. I have paid $15-$60 for a pullet in the past. I have seen rare, show quality birds sell for more. You will only ever get any money for a rooster if he is rare and/or show quality breeding stock. I would say $15-$25 for a pullet that is at POL, healthy, friendly, pet quality. Less if it's a production hybrid.
9) Grit should be offered free choice on the side, not sprinkled on food. They will eat what they need. They will pick different sizes. They will get some from their environment. It doesn't necessarily get eaten at the exact same time as the food and can stay in their system for awhile (I believe).